1. Technical Field
The present invention relates generally to plasma-producing devices, and in particular to electron cyclotron resonance plasma-producing devices. More particularly, the invention relates to electron cyclotron resonance plasma-producing devices employing a permanent magnet and shaped pole piece to control resonance zone location and plasma uniformity.
2. Description of the Related Art
Plasma producing devices are commonly employed in microelectronic device fabrication and similar industries requiring formation of extremely small geometries. Plasma producing devices may be utilized in plasma-assisted processing to etch geometries into a substrate or to deposit a layer or layers of material on the substrate.
One class of such plasma producing devices employs a magnetic field in conjunction with microwave energy. In these devices, plasma is produced from a working gas as a result of the interaction of a magnetic field with an electric field. A microwave waveguide may be employed to inject microwaves, which have an associated electric field, into an evacuable chamber containing the working gas. The microwaves propagate into the chamber in a direction substantially perpendicular to the surface of the workpiece. The electric field associated with the microwaves is perpendicular to the direction of propagation, radially outward from a line following the direction of propagation of the microwaves. Plasma ions from the working gas are accelerated by the electric field along such radial lines.
A magnetic field is provided in a direction generally aligned with the direction of microwave propagation, causing plasma electrons within the working gas to rotate around the direction of microwave propagation at right angles with the magnetic field. At the plane of resonance, the point at which the electric field associated with the microwave energy and the rotation of plasma electrons are in phase, the microwave electric field constantly accelerates the rotating plasma electrons. The energy of this acceleration dissociates molecules of the working gas into atoms and removes electrons from the atoms, creating ions and additional electrons. The ions then diffuse and impinge upon the exposed surface of the workpiece.
The requisite magnetic field may be provided by a single permanent magnet situated above the outlet of the microwave waveguide into the chamber. An adjusting element may be provided to vary the spatial relationship between the magnet and the waveguide opening, thus altering the location of the plane of resonance or "resonance zone" within the chamber.
While use of a permanent magnet situated over the waveguide opening to the chamber has advantages over other plasma producing methods, a permanent magnet of the size required to provide the requisite magnetic field may be very expensive. Additionally, plasma uniformity across the surface of the workpiece is generally necessary to achieve etched geometries or deposited layers having relatively uniform dimensions from the center to the periphery of the workpiece surface. Prior art attempts to obtain plasma uniformity have focused on achieving a uniform magnetic field, which requires very large and bulky magnets. Another drawback of the use of permanent magnets in plasma producing devices relates to the necessity of positioning the microwave waveguide between the permanent magnet and the workpiece. This constrains placement of the permanent magnet with respect to the chamber, and as the magnet face is moved further from the chamber, larger, more expensive magnets are required to produce the requisite magnetic field.
One solution to these difficulties is described in U.S. patent application Ser. No. 08/770,316, which teaches the use of a reduced height waveguide with a rectangular-to-circular transmission mode converter, allowing the face of the permanent magnet to be moved closer to the resonance zone. Since magnetic field strength is inversely related to the square of the distance (.about.1/d.sup.2) for a dipole, this placement of the magnet closer to the resonance zone allows a smaller, less powerful magnet to realize the same magnetic field in the resonance zone. Although permanent magnets and electromagnets do not provide magnetic field which are precisely those of a magnetic dipole, both provide fields having predominantly dipole characteristics. Closer to a permanent magnet, more substantial deviations from a dipole may be found.
The above-identified patent application also discloses the benefits of employing a return piece (also called a "pole piece") adjacent the pole of the permanent magnet which is opposite the waveguide and evacuable chamber. The return piece, which may be composed of soft iron, short circuits the magnetic field emanating from the surface of the permanent magnet adjacent to the return piece, increasing the "effective height" or perceived magnetic strength of the permanent magnet. The return piece compensates for magnetic flux which would be generated by a larger magnet, allowing a smaller permanent magnet to produce a magnetic field having the strength equal to that produced by a similar magnet (without the return piece) of a larger height. Since the magnetic materials are typically very expensive, the ability to utilize a smaller permanent magnet to produce the requisite magnetic field at the resonance zone improves the commercial viability of the plasma producing device. Employing a return piece also reduces stray magnetic fields beyond the return piece.
The return piece described, particularly with optional sidewalls extending from the return piece and forming a "sheath" around the permanent magnet, provides some ability to shape and direct the magnetic field produced by the permanent magnet, and to minimize stray magnetic fields around the magnet. Control over both the direction of the magnetic field lines produced by the permanent magnet in the resonance zone and stray magnetic fields may thus be achieved. Additionally, the magnetic field near the workpiece may be minimized, resulting in an inherent increase plasma uniformity.
It would be desirable, however, to provide control over shaping of the magnetic field to achieve benefits in plasma processing. It would further be advantageous to adjust field strength and shape through positioning and/or shaping of a pole piece in a permanent magnet assembly for a plasma-producing device.